1. Field of the Invention
This invention relates to devices for spray application of coating materials, and more particularly to spraying devices for achieving uniform application of a wide variety of liquid coating compositions to a workpiece in ambient conditions generally adverse to the spray application of such coating materials.
2. State of the Art
The advantages of spray application of clear and pigmented coating materials such as oil- and water-based acrylic primers, paints, lacquers, enamels and varnishes to various substrates are well known. Good spray-applied coatings characteristically result in films of uniform gloss and thickness without the undesirable aspects of streaking, sagging or blotchiness frequently produced by brushing, rolling or dipping techniques. On the other hand, spray application of coating materials requires more costly equipment, the use of which must take into account several critical parameters. For example, successful deposition of a coating material on a workpiece entails firstly that the atomization process, that is, the formation of discrete droplets of coating material, create a stream of dispersed droplets of the finest particle size possible with a range of variation in particle size as narrow as possible. Secondly, achievement of a deposited film of uniform gloss and thickness is dependent upon efficient transfer and adherence of the solid material carried by the spray droplets to a workpiece.
It is well known that transfer efficiency and film build-up are greatly affected by the relative humidity and temperature of the atmosphere through which the stream of spray droplets must travel. This is because a large portion of the coating material comprises a volatile component which must be driven from the remaining coating solids before effective adherence is accomplished. Controlled partial evaporation of the volatile component is desired during particle travel to the workpiece, but such partial evaporation is hindered, especially in the spray application of water-based coatings, by high ambient atmospheric relative humidity, and can be further hindered by too low ambient temperatures; too high ambient temperature, on the other hand, tends to reduce deposition efficiency by overdrying the droplets. If the ambient humidity is too high, partial evaporation occurs only to a slight extent which results in a deposited film characterized by mottling, sags or runs. On the other hand, if the ambient humidity is too low, partial evaporation may be excessive, thereby resulting in poor transfer efficiency and graininess at the film surface.
Hence, successful use of spray coating techniques has heretofore been largely limited to applications where the coating material is of the volatile organic-based type less affected by high relative humidities, or to applications where water-based coating materials can be applied under controlled conditions of ambient humidity and temperature. Because of the obviously high costs in providing a temperature and humidity conditioned atmosphere to house a workpiece to be coated, especially in the instance of workpieces comprising industrial equipment, vehicle chassis, or even a building, it has been much preferred to utilize organic-based coatings over the water-based equivalents.
The advent of spray coating devices utilizing electrostatic charging means to impart electrical charge to spray particles, by either corona-produced ion bombardment of the particles or by inducing charge directly thereon, has improved the uniformity and fineness of spray particle size and the efficiency of particle transfer to, and deposition on, the workpiece. Electrostatic techniques alone have not, however, overcome the problems arising from use of water-based coatings under ambient conditions of high relative humidity. Yet, the need for a spraying system enabling the utilization of water-based coatings under widely varying conditions of humidity has been accentuated recently by a combination of factors, including stringent regulations imposed by State and Federal governments upon users of volatile organic solvents, requiring such users to minimize emission of solvents to the atmosphere, and by increased costs of petroleum derived compounds such as xylene, toluene and methylene chloride typically utilized as solvents in organic-based coating materials.
One recent attempt at solving the aforementioned problems is described in U.S. Pat. No. 3,857,511 issued to T. S. Govindan on Dec. 31, 1974, which is directed to a process of applying water-based acrylic paint from a conventional air-atomizing spray gun, wherein a cone or shroud of humidity- and temperature-conditioned air is formed around a stream of paint particles. Because of the configuration of Govindan's air shroud producing structure, however, the aforementioned problems of spraying water-based coatings in an ambient atmosphere of high relative humidity remained to be solved. Furthermore, it has been found that a spray device having the Govindan type air-shroud producing means is particularly unsuited for use in combination with electrostatic charging means because of turbulence created by the angularly directed shroud air which increases rather than impedes deposition of coating particles on the electrodes.
Spraying methods are also useful in the manufacture of other products, such as the application of materials to glass to form tinted glass, mirrors or laminates. Where the glass substrate is in a heated condition, or where a very thin layer of coating material must be applied to the glass surface, application of material by spraying may be the only practical method for achieving a uniformly coated substrate. When spraying rapidly oxidizable coating materials onto glass substrates, it is often necessary to exclude atmospheric oxygen or moisture from the region of particle travel to prevent unwanted reactions of particle components with the ambient atmosphere, such as premature oxidation or hydrolysis, for proper coating deposition on the substrate. In the case of electrostatic spray coating of non-conductive substrates such as wood, plastic or glass, it may be necessary to ensure that the workpiece is made sufficiently conductive by controlling the level of moisture at the surface whereby excess charge of deposited particles is drained from the workpiece surface to the atmosphere. Exclusion of oxygen and a controlled moisture level can each be accomplished by providing an envelope of appropriately conditioned gas to surround the spray stream as it travels to the workpiece.
The gas envelope forming structure of the present invention may also be used in coating systems in which the dispersing apparatus is of a type used for application of dry or slurry-based powder coatings. The device is especially useful in dry powder coating processes wherein the powder is electrostatically charged to improve deposition. Frequently, when dry powder coatings are subjected to a high voltage field in an ambient atmosphere of low humidity, sparks may ignite the powder-air mixture causing dangerous fires or explosions. The provision of an envelope of humidified air or inert gas around a stream of the powder coating material practically eliminates any tendency for the powder to ignite.